Inkjet devices, particularly piezo electric and thermal, are common for both personal and industrial printing purposes. Most commonly, such devices are found in consumer homes as a means to create high quality prints and photos. In such applications the fluid inks that are commonly used for jetting onto various types of white papers utilize molecular colorants commonly known as dyes. Dyes have an advantage for such systems because they are typically more stable than pigment colorants. Pigmented, or particulate based colorants are challenging to stabilize in fluids used for jetting applications because such fluids typically require a very low fluid viscosity, most commonly less than 10 centipoise. For formulations of such low viscosity any particulates having a specific gravity greater that of the fluid carrier will have the tendency to settle over time, and this settling rate can be predicted for Newtonian fluids by Stoke's Law. Pigments are frequently used for black inkjet inks as they can be more easily stabilized by incorporating particles that are small enough, typically less than 50 nanometers, to be at least partially stabilized by Brownian motion.
If opacity, or hiding power, of the underlying substrate is a desirable property of the ink then particles of greater than 100 ηm are typically required. More specifically, particles less than 100 ηm are invisible to the human eye. The maximum efficiency of coverage is typically seen in particles greater than 200 ηm. Thus these particles will be large enough to be subject to Stokes Law. Furthermore, the material or composition of the particle is also very important as materials with higher refractive indexes will have greater hiding power. The challenge then becomes finding a high refractive index particle with a specific gravity as close as possible to common fluid carriers, usually between 0.8 and 1.2. The options are very limited and the most commonly used opacifying particles that have a refractive index greater than 2.0 also have a specific gravity greater than 2.0. In the case of titanium dioxide, the opacifier most commonly used in the paint industry, the specific gravity is 4.2, making it settle rapidly in low viscosity fluids.
For these reasons existing inks that contain high specific gravity particles like titanium dioxide are designed to be able to be re-mixable with some type of vigorous mixing, either automated or hand shaking. There are only a few examples of such inks in the marketplace, and they are all designed for industrial applications. All existing examples are delivered in a bladder type cartridge or directly from a jar/can. An example of a titanium dioxide containing white ink from Epson is the EPSON Ultrachrome® GSX White Ink Cartridge. This is used for commercial printing applications and utilizes a solvent based formulation designed for Piezo inkjet systems. The cartridge is designed with a large bladder reservoir containing the ink and requires weekly re-mixing by vigorous agitation. Another example is the Hewlett Packard Scitex white ink cartridge. This intended for use in HP Scitex series printers outfitted with an optional Scitex White Ink Upgrade Kit. The upgrade kit contains an HP White Ink Homogenizer that automatically shakes the white ink cartridge daily to keep the ink pigments in suspension. Thus current opacifying white ink cartridges are only available for industrial or commercial applications and all are designed to contain the pigment in an open reservoir type delivery system so that they are able to be re-mixed through shaking. These industrial printer examples are simply not compatible with consumer, in-home, hand held printing devices.
For example, attempts have been made at more precise, and localized application of compositions that hide, or cover-up skin deviations. Handheld devices that are moved across the skin have been developed to apply skin treatment compositions to local defects. But these devices have been plagued by a variety or technical issues including the difficulties associated with printing compositions that comprise particles as discussed above.
Therefore, there exists a need for printer compositions that contain particulate matter which can be printed on human skin, paper or any other surface. The particles must be reasonably stable in solution and easily redistributed into the solution should they settle. The particles must be big enough to be visible, but not so big that they fall out of solution quickly. And the particulate containing inks should be thin enough to be used with current printer cartridge and nozzle technology. Preferably, these particulate containing ink compositions can be used in methods and apparatuses that can quickly and precisely detect tonal and textural defects on skin. Then with equal speed and precision, apply the particulate containing ink compositions directly to the deviations. These compositions and apparatuses are defined by the present invention.